I have tested this and found the following...
1. if you have 15 licenses total and assign 10 to project1 and 5 to project2 then when a sixth person tries to login they will not be able to and will get a license error.
2. if you have 15 licenses total and you assign 8 to project1, 5 to project2, and leave 2 unassigned then when the sixth person tries to login to project2 they will be given a license and allowed to login (if the two unassigned licenses are not in use by project2).
So in answer to your question yes if all full licenses are assigned to domains then once the allocated full licenses are used up then no one else will be able to logon to the project and receive a full license.