This is described in each Brawl's window.

There are two ways in which you can get Brawl points from Presidios.

First, you can attack King Carlos's (i.e., unclaimed) Presidios:

Obviously, this has nothing to do with your troops in your Presidios. You just attack unclaimed Presidios and claim points*.

Second, you can defend from your opponent or attack their Presidio*. This is not specific to Presidios; it's just the PvP component of Brawls:

As all PvPs, this doesn't have a recipe, as it depends on your opponent's units. A simple example: if you attack a Presidio with only Subs on it (it won't happen in practice), you would do well to send Pirates and not Armada (look at Subs' stats: they are strong against Armada, but pathetic against Pirates). Similarly, if your Presidio got attacked by - for example - only Armada, you would do well to defend with Subs and/or Junks.

However, you don't know ahead what will they attack with, so you cannot plan like this.

So, neither scenario leaves any room for planning "which units to send to Presidios". You could plan a bit with your attacks if you scouted their Presidios first, but that's usually expensive in scouts losses (which translates to free Brawl points you're giving to your opponent).

* Please note that points are distributed among all the participants of mass actions (like mass attacks on a Presidio or defending one) and stronger armies will get more points. This is almost irrelevant for Brawls (points are counted for the whole BH), but it does affect your "activity" (i.e., the number of points that you earned for the BH).