There are plenty of pet mods: http://www.tesnexus.com/downloads/search.php?name=pet&scid=20&author=&mname=&desc=&size1=&size2=&downloads1=&downloads2=&udate1_day=&udate1_month=&udate1_year=&udate2_day=&udate2_month=&udate2_year=&ldate1_day=&ldate1_month=&ldate1_year=&ldate2_day=&ldate2_month=&ldate2_year=&endorsemants=&images=&readme=&advltonly=&opensource=&page=2&orderby=name&order=ASC
I think Guar are particularly cute though: http://www.tesnexus.com/downloads/file.php?id=20358
As long as the pet/companion character's level is calculated based on your level, they will level up with you.
For magic effects: I'm using Enhanced Magic Effects - http://tesnexus.com/downloads/file.php?id=9424
I tried, Better Quality Magic SFX, but I found them to loud. They actually scared me a lot because my character is an assassin-in-training, it was dark when I added the mod, and I was simply caught of guard by the change... http://tesnexus.com/downloads/file.php?id=35960
I haven't tried this, but I downloaded it: Poison FX - http://tesnexus.com/downloads/file.php?id=12568
Same goes for this, but the reason I have yet to try it is because it conflicts with a fire retexture mod I really like - http://tesnexus.com/downloads/file.php?id=32383
--Tomlong75210