--- What video format should i first convert these videos all too to ensure that videopad is going to be able to edit them optimally ---
In my experience, .avi is the universal video format, as .wav is the universal audio format.
But there are other things to worry about, mostly frame rate and the size (as viewed) of your different videos. Mixing different frame rates, or different viewing sizes, is unlikely to be successful.
If you must convert frame rates or size, probably your best bet is VirtualDub (Google for it). It's been around for years and is something of a de facto standard in video editing. VirtualDub does not have effects, but otherwise it can open, edit, and convert almost anything--almost.
There's a learning curve to VirtualDub, but YouTube has a lot of help videos--some garbage, some not, as usual.
Hope this might be of some help.
-al-