I know I had a really bad problem with kids taking mine out of the holster at an event. I tried the velcro first, (they sell very thin black velcro and you can't see it where it's placed) but the velcro was so strong I would have to yank on it to get it out. So then I went with the strap. I used some scrap white leather, rivit and a snap...works like a charm.
Once you put the armor on, it should look fine. All of the elastic will hold everything in. As for the back pocket, if you're going to put the back flap on you may not need to remove it.
I'm 5'4" too and I bought the SC kit. I really didn't need to trim too much, mainly the shoulder bells (just too big all around), but that's an easy trim. As for the rest, only a little trim here and there.