You are welcome. Sorry if you already knew a lot of what I posted.
Some of the things I see on TV in the 'bit and tack' department on international showjumpers absolutely amaze / horrify/ baffle me!
Once, as a 'challenge' I managed to identify and describe the use and action of, it must have been getting on for a hundred different bits. NOW there are 'mylers' and 'neue schule' and 'mikmar' and all manner of new bits which I still have to learn!
In the UK, a tom thumb is a totally different bit! Only in the USA have I heard the term 'tom thumb pelham'! Just a confusing difference between continents, I think!
When I ride one of my horses in a pelham, I use predominantly the snaffle rein for collection and bend, and if the horse gets a little 'above himself' (sometimes literally!) I use a gentle tweak of the curb rein to say 'oi you'! Which enourages his head down and for him to round.
Often, when hacking (another confusing difference in continents - hacking = trail riding, I have discovered!) I knot the curb rein and leave them on my horse's neck, as my 'emergency brake' just in case he gets a bit 'psychotic'!
Be careful when schooling in a pelham that you don't use too much curb rein and create 'overbend' rather than 'roundness and outline'. It's very easy to think 'oh, this is nice, the horse is nice and rounded' and in fact the horse is overbending and going behind the bit.
Regarding mullen or low port, I really think it depends on what your horse likes in his mouth. Trying the mullen first is generally a good place to start.
You sound like you have a pretty good idea anyway what you are doing, so good luck in trying. If you need any more help, I'm happy to do my best.