What filtration is in the little tank? You will want to do a couple partial significant water changes a week or the adults are at risk of illnesses.

Remember the fry in the covered jar, 1/3full with the water from their previous home? Assuming that the 10-gallon is healthy, add as much of that water to the jar as is already there. COVER the jar again - ask me why I'm keen on that. ;)

After about 20 minutes, cover the jar with your hand and pour about half the water into a bucket. (If you pour to a reasonably clean bucket you can retrieve fish which could be lost if spilled into a toilet.) Add as much water from the new tank as isthere. Let it sit 20 more minutes and decant. Do that a third time, if you feel that the two tank's chemistries were really different.

Now your Dad may be perfectly correct if you frequently did partial water changes on both aquariums. Because of populations, eating and metabolism and the size of the tank, the fry tank probably was more inclinded to drop it's pH faster than the ten. We have a bit of a fudge factor when moving fish into tanks a little warmer than the previous or a little higher in terms of pH or a little higher in terms mineral content - of  DH or to a degree TDS.

If they were pretty close, than your Dad certainly, as a member of the "32 feet per second acclimation school," is also right.

I guess its a matter of how careful you may want to be.

Elsewhere, someone noted that if you open a bag of new fish or fish from the mail and the water smells really bad and you can assume that they regurgitated their food, that slow acclimation approach can be way more dangerous to the guppies (or whomever) than quickly getting them out of the filthy water (all of which is dumped through a net into that buckker) and into clean, safer water. That wise point is one I really hadn't considered. DUH! yet once again! :)

Would your Dad be comfortable changing the guppies both of those ways?