Joining a second jamaat of musaafirs for Asr Salaah

Q: I went to a musjid late, when asr namaaz was over. However, I noticed a second jamaat taking place in the sehen,so I joined it. I got the last rakaat. So after salaam I completed my 3 rakaats. Now, after a few days, I discovered that the jamaat namaaz which took place was by musaafirs, so they read 2 rakaats, and I completed 3 rakaats, what is the condition of my namaaz? 

Reaching into sajdah before the Imaam

Q: It takes our imam a bit long to go into sajdah, standing up from sajdah, etc. So what happens, for example, is that after qaumah, as the Imam goes down into sajdah, he says Allahu Akbar, and we also go into sajdah, but some of us reach sajdah before him. Is his and our salaah valid? Should we request him to rather try and do his postures slightly faster? I think it is because of his weight problem.