I want to add custom ringtones and notifications.
How to I put these on the phone so I can choose them as ringtones?
How to I put them on the phone so I can choose them as e-mail/gmail notificaitons?
And finally, same question but for texts?
As with all Android phones, simply connect the phone via mass storage mode, browse to the MB860 "drive", click on it, and create a new folder off the basic root drive portion on the SD card or internal memory (USB mass storage mode) and label as so:
media/audio/ringtones
media/audio/alerts
media/audio/notifications
Then just drag and drop the MP3 sounds you want in each area and once you disconnect the phone, it will "mount" the SD card and/or internal memory and magically find your new ringers, alerts, and notification sounds. Open up the Settings, Sounds, and apply them as you see fit.
Quite easy compared to Apple devices. Just plug and play really.
What are the size/length restrictions?
I'm not 100% on those, but I do know that up to 320kbps bitrates on MP3 works just fine (sounds MUCH better than the crappy 96 or 128 you get from online sites), and I don't think there is a specific size or time limit. I set my MP3 at 320kbps and 30 seconds as most carriers transfer to voicemail after 20-30 seconds.
The alerts (for alarms) and notifications can be longer from what I've gathered, but there isn't much need beyond 30 seconds. If you're not awake after 10 seconds of "AC/DC Highway to Hell", you're not with us anyway.

You didn't ask about music, but I just create a folder called Tunes and drop all the songs I want in that folder. Once I mount the drive, the phone finds and plays any MP3 320kbps bitrate songs I have on the device. Easy stuff...