Will the Tascam DR-40 add any self noise or hiss to higher quality external mics? Also is it okay to use two different external mics into the Tascam DR-40 at once? Thanks
Every piece of gear will add its own self noise to a system. Whether that noise is perceptible is another question which depends a lot on the gear, what you're recording, and the settings you're using.
For example, if you're recording a very loud source (eg. A rock concert) you won't need to set your gain very high and so less self noise will be added. If you're recording quiet sources then you generally want to set your gain higher which will amplify the recorders self noise along with the self noise of anything in the recording chain before it.
I've never used this particular recorder but as a prosumer device I can't imagine it to have the most high quality preamps and so will introduce a high level of self noise compared with other professional recorders.
As for using different microphones at once, I don't see why not! Unless of course you're trying to make stereo recordings, in which case you should be using the same model of microphone at the least (unless it's an MS recording), but that's a whole new topic of conversation!