Whether or not you need more than a 75 gallon depends on the sex of your RES and how big s/he grows.
Since you said you have both tanks already, you could put your turtle in the 29 gallon tank and move it to the 75-gallon tank later, but you'd definately have to upgrade your filter. Your turtle will grow fairly quickly if you give it good living conditions, though, so you may just want to use the 75-gallon (up to you).
Don't know what you mean by "over filtering the 75-gallon," but for adequate filtration you should get a filter rated for 3X the size of the tank (the specs on filter boxes are largely for fish, which aren't as messy as turtles are). More filtration never hurts. If you choose to use the 75-gallon and have the water somewhat deep, make sure your turtle has places (like floating plants or ledges) to rest on in addition to the basking area--little turtles can get tired when swimming and need places to rest.
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-