Brown University says that the ACT scores will suffice in place of SAT+SAT lls. My question is that whether sending ACT and SAT lls will be benificial or not? Am I allowed to do so in the first place?
If you have favorable scores beyond what's require, it can help slightly to send them. After all, admissions officers are people too and will get a good impression by just glancing at them even if they don't officially "count" them.
As you mention, the Brown website says you do not need to submit any SATs if you submit the ACT.
However, I would in fact encourage you to submit at least SAT II (Subject test) scores if they're strong. SAT Subject tests show very different skills than the SAT / ACT, and could be an advantage. This may lead to a slight advantage.
If your SAT I score is very strong as well (e.g. stronger than your ACT), I'd send it too just in case for a very light advantage.
Of course, since this is in addition to the Brown requirement, neither of these are needed, nor will the help be massive.