You are correct the sand would be too small for the UGF. I have one a lot of people here don't like them very much but thats difference in opinions. You have to use a certain size gravel with a UGF one thats not too small and one thats not too large. Sorry I am sure someone knows the exact size the gravel should be.
i don't think they do a good job, also, they disrupt plant roots, and you have no option of what filter media to use...it's your choice, but i'd opt for an internal or two
The problem with undergravel filters is that they collect more and more waste under the filter plates and cannot be cleaned without tearing apart the entire tank. Over time they start to degrade water quality.
Although UGF make for excellent biological filters, power filters do just as good a job while also performing mechanical filtration. In addition they are very easy to clean out.
I have a power filter and a UGF both also. I love em but like everyone said it's your tank and you should do what you want with it. I could have saved money by not buying one because the power filter does a good job alone with the bio wheel. I had a UGF a few years ago and I liked it. As far as plants go mine are growing very nicely with it.