Hello Kris,
My daughter and I have been running a planted 35g hex tank for a couple years or so. She likes the fish and I like the plants. 29g is a great place to start and get your "feet wet"...
I think it is really cool that you are raising native Mollys! If I had access to native mollys I would have a tank full too. You sound like a DIYer yourself. I think building pieces for my tank makes the hobby a lot more fun.
I really love sumps and even have a mini sump on our 35g hex. There is no minimum size of tank that is required to run a sump. I would probably even run a sump on a nano tank. There is nothing wrong with canisters, hang on back filters or even in tanks filters... they are very relible filters especially when you are starting out and have so many other things you are worrying about.
We had a 110g glass tank several years ago. It sprang a small leak... probably due to the lousy pet store particle board stand that came with the used tank not supporting it sufficiently. I was in the process of building a replacement stand when the leak happened. I was too anxious to get the tank up and going to finish the stand before setting up the tank. Leaking tanks are not a fun experience and not something I wish to repeat... I tend to over build pretty much everything I do. Sounds like you do too, an angle iron stand should last you MANY years without issues.
Welcome to the site and the hobby, it Is a lot of fun... with some moments of heart break thrown in to keep you on your toes.