Its all a matter of personal preference. If you don't want to have 100s of apps, thats your choice. But, I have no doubt, that users on this forum have that many apps loaded on their devices. As long as the app fits on the SD memory, you can choose to load it. And even that may change now that the phone has been rooted.

 

Just to clarify all Apps are stored on the phones internal memory. Parts of the app can be stored on the SD such as map tiles or pictures but the app itself is on the phone itself.

 

As long as there aren't more apps running at the same time, no.

That's like asking if software that you never open on your PC is slowing it down.