I have composite fillings in most of my teeth, including the front. can i safely use a dental trainer for teeth alignment, or can it cause cracks?

I don't know: What you mean by "a dental trainer". If you mean a removable orthodontic appliance such as a Hawley or a tooth aligner like Invisalign, the answer is yes you can. Any loose composite fillings should be replaced before the teeth re - decay.

Aligned can be great: Aligners should be gentle enough for you

Fillings and...: Generally speaking, if the composite fillings are sound, then Orthodontic movement by any number of methods should not harm them. If they break or come out they were probably in need of replacement or the teeth should have some more permanent or stronger replacement. Having said that the dentist actually treating you is best to make that determination. Discuss with him or her before treatment.

See a Pro: An Orthodontic Specialist can evaluate your malocclusion, determine the best course of treatment, choose the appropriate appliance with you, and monitor progress. Without an examination cannot determine health of your restored teeth. Be wary of over the counter gadgets, as well as treatment by a non-specialist. In the long run best treatment provided by the most skilled specialist.