Well first I always try to use the industry standard definitions and not invent terms.
http://mmoterms.com/full-mmorpg-terms-glossary
Alt: Short for “alternate”. It refers to the alternate character a player has from their main character. This is not a stable category as sometimes alts can out level mains and sometimes mains become moth-balled.
Newbie: A new player to a particular game. Newbie is usually the nicer version of noob or nub
Noob: The pejorative form of newbie.
I really don't like calling someone a noob unless their behavior suggests they have no clue what they are doing. So a newb to CH may not be a noob. Some people never stop being noobs because of how they play. (These are the guys you are never let into/get kicked from better clans.)
The word Main suggests a toon that is played often. IMHO if you are willing to take a toon through the grind phase of the game that qualifies as a main. I know people who play enough that they have 5 mains.
For the lower or mid level toon I just prefer "Alt" because it suggests a toon that is not played as often. I know people with no main and a few alts. Some people would accuse me of only having alts.
So for what we are discussing I would suggest we just call them "alts" or if you want to be more clear, "mid alt" or "low alt". Newb would also work but it assumes that the low level means the toon hasn't been around very long.
A nice test is to use the terms in a sentence and see what impression it gives you.
"The castle is full of alts today." It suggests ... the castle is full of toons with names you don't recognize or mid levels.
"The castle is full of newbs today." It suggests ... the castle is full of lower levels.
"The castle is full of noobs today." It suggests ... the castle is full of clueless idiots.
Personally I really like the quest line up until Tam Lin and then I loose interest. Thus I have a lot of low and mid alts. I only have one toon I consider a Main because my Mage is the only one I have taken the time to grind over 120.