Easier SPS
 

I have a T5-lit mixed reef: softies, LPS, BTA's, and for SPS I have Montipora digitata and Montipora capricornis.

I have no calcium reactor, I do kalk plus DIY 2-part, I am not very rigorous about 2-part additions and water testing, my calcium and Alk level are usually pretty low (Ca anywhere from 330 to 380, alk usually around 7). Despite this the Montiporas have thrived.

What other easy SPS would be good to try?

Also has anyone had success attaching corals to the back wall of their tank or overflow box? I am running out of real estate in my tank.

I'd say once you get your Ca & Alk in check you could go with some encrusting monti, green slimers are pretty easy to care for, they colour up nicely and grow relatively fast. Blue tip staghorn, or staghorn's in general are fairly hardy. Some may disagree, but I find Millepora's to be fairly easy to keep, they grow fast and again, colour up nicely.. Another easy and fast grower is a birdsnest.. they are really neat once they are grown into colonies, they grow super fast. That's all I can think of for now...

I have kept all of these SPS and more under 150 & 250w HQI , in a 20 gallon tank. I am pretty sure that the T5's will be sufficient for the majority of these SPS..

I am sure you can have corals like montipora digitata, caps, staghorns, etc could be grown off the glass. I'd use epoxy as I doubt super glue would work that well. Once it starts encrusting then it will branch out. I have never tried this though so don't take my word for it haha.