If you have the power filters, maybe you should start just the 55 going. You could hang both of the power filters on the sides so they don't crash. The hex could be colonized a couple of days later. If it looks like you are going to be able to add over 50% of the water in the new tanks (especially after gravel, ornaments and accessories are added), go for it.

This assumes that the chemistries of the two tanks are fairly similar. If you have the same water source, a similar decor, much the same and militant partial water changing habits and tanks which are not too crowded (lots of waste material often significantly drops pHs), than prolly isn't a big issue.

If you have a few fish in the two tanks, which would not be compatible with the others, could you separate them into the hex? So long as the tank is stable in terms of temperature, you could skip feeding a couple of days (or if it is for fry, feed lightly) until you have walked up the water. You didn't mention the gallonage of the hex.

You know this, but for someone passing by and reading this, hexagonal tanks usually have much less surface area to gallonage than most tanks. They will support correspondingly fewer fish.

You probably will be lucky to get 30-35 gallons of relatively "clean" water out of the other tanks. Remove the water to the new tank before doing anything else, except unplugging the power filters.

I use power filters so little that others will probably have better advice than I would. But I thing I would drain the power filter (first) into a slop bucket to get rid of extra detritus. Then I would gently, but immediately, refill them with tank water.

The dirtiest water in a tank is on the bottom and often at the surface. Siphon from 1/10th down to maybe 7/10s or so down.

Then, if you have a bucket, put some of the remaining water in there, net out the fish, gently drop them in the bucket and cover it. That may sound odd not just putting the fish in the new tank, but they are sensitive to the fall of gravel. The bones in their ears are quite similar to ours, they just don't have the funny earlobes. ;)

Also, while they are in the bucket, some of them will nervously pass urine through their gills and defecate and regurgitate. You will move them to the tank later. Then that "stuff" in the bucket of water might just as well be tossed into the flower garden or down the ceramic file.

Until mentioning the above, I really hadn't thought about it, but maybe (just as with the preparing fish which will be shipped) don't feed them for two days before the move. So if this is to be done on the weekend, stop feeding them now. Despite what they are "telling" you, they will do just fine, though bite-sized fry may be at risk.

Yours is a good question about rinsing the gravel. Sometimes the amount of "stuff" in the gravel is just amazing. Once the fish, plants and accessories are removed, start in on the gravel. If you can grab and pour the gravel through a net, maybe most of the unwanted material will just pour through. Slosh and lift it out. You will know whether it needs a light rinse or can be moved, as is, to the new tank. In light of the issue you raise, you might gravel vac a few days before the change over.

When seeding a new tank, I take out water from my "start-up" tank first. Then I net out some "starter gravel" and let it rinse in a net. Always the remaining water will be a little messed up, but I don't want to tote that detritus into the new tank. And it settles, by the time new water has been added to the set-up tank.

However, there is a school of thought that suggests that a little "clean dirt" is useful in starting the new tank. I fear my tank dirt can beat up any other dirt in the universe and don't wish to take it anywhere but the garden. ;)

When the old and new water and gravel are in the tanks, landscape (seascape) the gravel, adding any rocks and the plants. There inevitably will be a little clouding. It should settle.

You again know this, but planting tanks involves gouging/ poking a hole in the substrate (gravel, sand...) with one's hand. The plant is set in the hole and gravel is gently cascaded in over the roots. Plants, which have some sort of node or thickened stem where leaves come together or the growing tip of a bulb should not have that part covered.

When adding the fish, be as gentle and quick as possible without getting them too worked up. I know that sounds contradictory, but some active fish have been known to die of shock. If we miss and miss a fish and chase it all over, that is not as good as a quiet ambush.

If you have an extra net, both for the initial catching and later transfer, it can be handy. Sometimes the only way to catch certain fish is to  carefully herd it/them into the second net with the first one. And the whole aquarium isn't stirred up by the chase. That works not only with the "blue streaks", but with the big guys like foot long pleco types.

If you have catfish and no second net, you might buy a fine-meshed net to drive them into. While the net may get punctured by a spine, the catfish will "hang up" less and get stuck far less in the fine netting. Better that the net gets torn up a little than the catfish. And if the process is smooth, the net may still be in great shape anyway.

Leave the lights off the first day. Let them hide in whatever cover is available. Maybe give them a treat of their favorite live food after they have settled in. Don't feed them much at first though.

I'm sure the shift will be work, but I'm also sure it will go well.