We peer direct where it makes sense: 
- if we see your networks behind our transit 
- if there is some traffic to exchange or particular request
We do not care so much about ratios and/or traffic volume that we need to carry to reach the peer. 
We usually don't peer with a customer of an existing customers and/or peers.
This has political reasons as it may put existing peering relationships at risk. 
Our MTU size is 1500 by default.
For our and your security MD5 accepted