1. Absolutely! Highly encourage it personally as it can help take some of the load off you in a very stressful day if they have them. The only people who tend to be truly against this philosophically are those who grew up with strobe tunes at the front of the band hall, or are just so 'they must use their ears only.' There are NO UIL rules against this!

2. I would not spend long on a warmup on stage - that's what your warmup time is for. Playing very briefly (like 20-30 seconds max) can be understandable. Play between pieces can mean a lot of things. Personally I have tuned between my march and a lyrical piece that's second to really dial things in (stage is a different temp, kids are going sharper, etc.). Please do not sit and PRACTICE pieces between pieces. That should be obvious, but I was judging with someone who had a school do that recently. If they don't know the music before they get to UIL, they sure ain't learning it that day!

Just keep in mind, if you take time to tune between pieces, that should not be you as the director tuning 5-10 kids individually. That only makes it more apparent where a judge needs to listen to. Also, if you spend time tuning on stage/between pieces, you'd better make certain it's getting better! Nothing worse than watching someone just flounder and make even more of a mess of tuning. (Individual tuners make this more plausible in my opinion as long as kids are taught to use them)