Yes, there is a cross tolerance between all amphetamines and if you use prescription stimulants like I do and want to use recreational stimulants you will need to take a higher dose than someone who doesn't take the prescription ones.
Yes. There is a very great deal of cross-tolerance among the class. If you weren't already aware, the Dexmethylphenidate (Focalin) is a weaker drug than Amphetamine.
Yep there is. For me using amphetamines for a while and taking methylphenidate soon after will reduce greatly the effects of mph, also the side effects are more bearable (anxiety, irritation) but also less enjoyable and more just a plain stimulant without any euphoria (not Even IV).