I am very into doing bonsi and I was wondering what sort of trees I could "bonsi down" to scale for a cham cage that would be safe? Also has anyone tried this?
Virtually any plant can be a bonsai, with root pruning, confinement to smaller pots, restriction of fertilizer, and pruning of the foliage. Citrus trees would work well, but they produce thorns, which must be clipped off. Myers lemon trees produce very few thorns and do well in containers. These are high light plants.
Miracle fruit is a shadier growing, small shrub that would do very well with a chameleon. Ficus, aralia, tropical fruits, like mango and starfruit....your options are limited by budget, commitment to the plant's needs, and your lighting.
I have seeds for a wild lemon tree that I know has thorns would it be toxic? I think I have heard that they are I'm not going to use them just in case though
No, that species is not toxic. Citrus are safe as far as toxicity, but that trifoliate orange is much too thorny. There are ore suitable varieties that have far fewer, easy to remove thorns.
Depending on what species of plant you want to "bonsai" or stunt in order to fit into your cage, you need to pay some attention to the size of the pot. If the species will need a very small pot to restrict its growth so its fits, you might end up with a plant that isn't stable enough for your cham to climb around in. Very shallow pots won't provide much anchoring.