The swiss travel pass in a nutshell is like a temporary GA subscription for tourists. Hence it is valid and only valid on the so called GA zone. I do not know which EuroCity train you will take to go to Venice. (As from what I remembered there are two lines extended to Venice, first one from Gevene via Simplon line, entering Italy via Brig - Domodossola. second one will be the san gotthard line from zurich to milano centrale and then venice) And if you are talking about travelling from Tirano directly to Venice on italian soil, that sounds crazy to me unless here is a direct bus by STC. (as you will need to take a nearly 3 hours regionale to milan and then change to venice, i predict a 6 hours journey also it will not be included by your swiss travel pass, obviously…)
If you will take the simplon line train passing through Geneve, vaud, valais for Italy, you can travel until Domodossola (your first italian stop) as it is included in GA. You will need to book Domodossola - Venice with trenitalia, if you book in advance you will be able to find very cheap ticket at 70% off (I visited Verona using this service for 12 eur) and then you can travel on the same EC train until venice:-)
If you are travelling on the gotthard line the STP will be valid to Chiasso. in this case I’d rather recommend you to get off at chiasso, and use the regional service to go to milano centrale (cost 5.8 eur, and 2 min faster in term of travelling time than the EC and the EC will cost you 12) then book your venice trip with Italo, a italian low cost + high speed train company, if you prebook your ticket with their voucher (very easy to find around) and like 2 weeks in advance or so, you will be able to find your train at 9 eur. and travelling time will be only 2 hours.
Hopefully you will find a solution that is good for you:-)