Hi Melisa,
1. How can I make our custom framework be the default selected framework when adding a new record?
I think you can't. As far as I know your only choice is to modify the default framework according to your needs. If you have someone in your team who is comfortable with SQL, he/she may be able to make a backup of the default framework (called kohadefault, say) and then to copy all the settings from your favourite framework in the default one. That being said: a. I may be wrong since I'm new to Koha. b. If I'm right you may want to submit an enhancement for that.
2. Is is possible to hide items on the OPAC based on itemtype? Do I have to modify each biblio one by one via the OpacSuppression feature to accomplish this?
That I don't know. Very interesting question, though.
3. Is there a way to limit the number of characters shown in the biblio summary when doing a search in the staff side? For example, if a biblio has a really long summary, we only want to show the first 3 or 4 lines.
Don't know either, I'm sorry. Kind regards, Sébastien.