Yes, one of the neat things about Lasalle is that the size or number of figures on the base has no influence upon game play. Heck, you can even play Lasalle with colored wooden blocks if you like, which could look good if done right, but most gamers like the visual appeal of miniature figures. Now, what size could those figures be depends on the individual or gaming group. The scale I have seen the most is 15mm figures, but 25mm figures are still very much in use.
In regards to 15mm figures. When did 15mm equal 18mm? It's amazing that some figure manufactures (and even gamers) still call an 18mm or 19mm figure a 15mm one. That's like saying a 30mm figure is a 25mm figure... but people do that too :-) AB, which produce some very nice figures BTW, used to call their 18mm figures 15mm ones. But now they refer to them as 18mm figures, as they are true 18mm figures. A trend I hope spreads, because if you are a new comer to the hobby, this whole "larger 15mm" phenomenon can get a bit confusing I would think.
Back in the late 70’s and early 80’s, figure manufactures (Heritage, Minifigs, and others) produced true 15mm figures. That is, the figure measured 15mm from the bottom of the feet to the top of their head (not including base thickness and the height of the shako). However, at one point a scale creep phenomenon began to appear. That is, figure manufactures began to make their figures bigger and bigger so they would not fit in with the earlier true 15mm figures, trying to force you to by only their figures. This is a result of greed and not team-work, which of course hurts the gamer. Some companies try to justify this by saying the measurement is from the ankle to the eyes, or something silly like that. But the true distance, and a lot of early gaming miniatures used this as a standard, is to measure the distance from the bottom of the feet to the top of the head; excluding base and shako. And this makes sense! When you go to the doctor, he doesn't check your height from your ankle/shin/knee to your eyes/nose/chin, he checks your height from the bottom of your feet to the top of your head, like normal.
Just for the record, even though I have many true 15mm figures, I actually like the bigger 18mm figures (I'm still a 25mm gamer at heart I guess), but I don't call them 15mm figures. I call them what they are. That is, if they are true 18mm figures, then they are 18's.