Every plugin you add to your server makes your servers performance go down a little.
So adding (loaded) un-needed plugins is not advised. However the only way to know for sure is to try it. I personally prefer sourcebans + hlstatsx way over mani admin, but then you do need an external web host so mani is a rather good option until you can afford a web host.

I used to run both, in past it was not a problem for me. I'm unaware if things have changed or not.

Hi,

Yes, La Muerte is right. Using Mani and Sourcemod same time will not block anything, but more plugins means more stress on your server. So if you don't need mani admin, you shouldn't get it.

But if you like it and want to install it, you can do that exactly the same way as without SM or MM. We also run Mani and SM on our server currently, and it doesn't cause any extra lag.

Still, if you have both, SM and Mani running with default/full settings, then you might have one problem with some commands. And with problem I mean that many admin commands will multiply when you activate them. So, for example, if you talk with default admin chat (in chat with @ say) you can get two messages, one from Mani admin and one from Sourcemod. Also some votes, like rock the vote (if plugin loaded from SM) will cause both admin plugins to answer.
So to avoid this, you can a) edit mani admin plugin (cfg/mani_server.cfg) or b) (which I prefer) unload all SM plugins with command "sm plugins unload_all" (without " ") (you could put this to your autoexec.cfg), then load needed plugins (wcs_effects.smx?) with command sm plugins load pluginname.smx (for example sm plugins load wcs_effects.smx). Just put that AFTER the sm plugins unload_all command in autoexec.cfg. You can find all the SM plugins in cstrike/addons/sourcemod/plugins and the autoexec.cfg in cstrike/cfg.

Low Pwner, since wcs requires SourceMod for effects anyway, people can just use sourcemod admin, which is just as easy and less exploitable as mani admin.