Unified Threat Management - UTM devices, proxies etc

In the context of an outage of our Cyberoam device, and the general decrepitness of the college proxies, I took a look at some available options, tuning etc.

https://www.quora.com/Which-is-the-best-Open-Source-Unified-Threat-Management-System-UTM-in-the-market

mentions ClearOS

http://jonmoore.duckdns.org/index.php/network-security/32-open-source-utm

reviews pfsense , untangle and sophos. He has an additional VPN requirement.

Comodo offers a free download.

https://www.google.co.in/search?q=set+up+a+unified+threat+management+appliance+with+open+source+software

For squid tuning, cache size etc,
https://forum.pfsense.org/index.php?topic=24429.0

where a user suggests, for an admin who has trouble with 70 users,

 I recommend use 10KB to 16KB(thumbnail picture size) for object size.
On restart COSS contents will be lost(developent still on progress for this issue)

for your memory issues:
squid/lusca would use 10MB per GB of total cache(when full).
http://wiki.squid-cache.org/SquidFaq/SquidMemory#How_much_memory_do_I_need_in_my_Squid_server.3F

If your system starts to use SWAP file(due to lack of RAM) then it will definitely slows your internet.
Squid/lusca starts to grow Page faults it also affects performance.

You might need to consider changing your cache_mem and give them to your OS.
This FAQ would help you.
http://wiki.squid-cache.org/SquidFaq