I know that AMXX has a built in feature for this, and seeing as sourcemod is more or less developed by the same dev's as AMXX I'd imagine they have a similar process.
For AMXX you would go to your addons/amxmodx/configs create a new folder called "maps".
In the newly created maps folder create a file called melee_extreme_v2.cfg (or whatever the map name is). In this folder you can change the cvar's, so if you want to have low grav, or change the timelimits, etc. Now, you can also load plugins this way. The way to load them would be simply to put the name of the plugin in there.
An example of lets say scoutzknivez.cfg
Code:
scoutzknivez.amxx
realnadedrops.amxx disabled
round_money.amxx disabled
nadegiver.amxx disabled
sv_gravity 250
In this example the file loads the plugin scoutzknivez.amxx, and disables the realnadedrops.amxx, round_money.amxx, and nadegiver.amxx plugin. It also changes the gravity to 250 instead of the default 800.
Now you can also create files in this folder like "plugins-cs.ini" Basically this folder you can enter any plugins you wish to load or disable and it will load/disable what you have listed on
any cs_* map. So cs_office, cs_italy, etc. All of these maps would read this file and go through and disable/enable any plugins listed in there. You can also do the same with "prefix_cs.cfg". This file you can enter any cvar changes you want to happen on cs_* maps. So you want low gravity on all cs_ maps? Go ahead and add it to this file
So with that said, try the same as above, however obviously the plugins aren't *.amxx, so follow suit with *.smx, and naturally the path would be /addons/sourcemod/config/
Let me know if this doesn't work. I have yet to try it on sourcemod, and have a feeling it may be structured differently, but it's worth a shot