Yes, indeed, your bridges have a limited number of stations related to profession. Those are, be default, occupied by NPC crewmen of the adequate profession. If you have four science stations, as you gain science officers, you'll see those filled. The actual position of the respective Boffs between those stations will vary.
However, the catch it that if you have 4 alloted science stations, but five science Boffs, those whom have seniority (that you've had around the longest) will be those showing up in priority.
So, if you've earned yourself a new Science boff but that you have already four others ahead of the new boff and badly want to see that Boff end up on a bridge seat, you're likely going to have to displace (dismiss/trade away) one of those in order to give the new Boff the wiggle room to be included in.