Hi there. Don't think there's too much question here. I would be of the opinion to go with what you already have. It makes financial sense for starters, it might be a bit of a config nightmare later on, buy you have the benefit of knowing your current systems config so you should be well aware of the requirements prior to proceeding. It's gosh darn good practice to "never place all of your eggs in one basket" :) Yes, it would definitely represent the best practice to go with what you already have implemented and pick up some polling hardware and spread it evenly throughout your current network.
I represent a Corporation with a configuration slightly similar to yours. Perhaps a few more servers, but only slightly. We have just over 30 servers in a hybrid networking environment. However, we currently use a SAN, for our databases and larger file servers, the largest file server weighing in at 170+ GB. We use a StorageTek 9740 Timberwolf Autochanger with 9840 Tape Drives. We are currently running this config with Legato Networker version 18.104.22.168 Build 112. I would recommend a TON of research prior to going with Legato as your solution, unless you are to run it from your Sun Solaris or Unix system. :) We run it on NT 4.0 at present and it's been a nightmare so far. :) I cannot say enough about the StorageTek or the 9840 drives though. We are currently backing up around 300 GB worth of data in very close to 2 hours. Not too bad over a 100Mbps Ethernet Net. When it works, there's nothing like it, perhaps sky diving though. :)
Anyhow, wanted to share.