The signal for the two Tampa stations increases nicely with height, so there's a good chance to get both of them with the added height and a fixed Y10713. You can start out with a 10.5' chain link top rail - cheap at the home center - for a mast on your roof peak - that will get you to 40' and see how it goes.
Both Ocala and Gainesville do not change much with extra height ... not promising. I would forget trying to get the Gainesville station - it's just too weak. Finding ca 24 dB of antenna gain just isn't practical without maybe a big antenna array or something like the fabled rhombic
http://en.wikipedia.org/wiki/Rhombic_antenna ... you think the 20' mast has a low WAF...
Easy to do a test and aim your existing 91XG at Ocala and see if it comes in. If it does, put a second 91XG on the same mast as the Y10713 and direct the combined preamplified and UHF-VHF mixed downlead to an A/B switch or a stand-alone tuner like the Homerun HD. No rotator needed, and optimistically you'd get three of your four target stations.