You can use for all the DB website or navigator app to plan and then check via website of the train company (in one station can more companies arrive and departure), or do you want a departure board?

you can look at interrail.eu website and you get most Information about every country (which companies, where you need a reservation). Most informations about if trains are part of Interrail you get via Railplanner App (but it is not up-to-date, because it runs offline and it gets only some times updates and there are some errors). 

To plan I would use DB website or app. I see that you are from UK, so Eurostar to Brussel needs an additional reservation (30€), From Brussel to Paris you can take Thalys which also needs an additional reservation (about 30€). Paris GDL to Zurich HB you can take TGV Lyria (35€ reservation). 

Zurich-Vienna Railjet express service with optional reservation 3€/seat via ÖBB website or with the Nighttrain Nightjet from 14€ for a seat.  

Vienna-Ljubljana Eurocity service optional reservation 3€/seat via ÖBB website.

Ljubljana-Venice you take a regional train to Trieste and change there for an regional express (RV - regionale veloce) to Venice. No reservation possible.

 From Venice you take an Frecciarossa highspeed train to Rome (10€ compulsory reservation, or nightrain 3€)

Really helpful, thank you! Could I just ask, what’s the point of an interrail pass if it costs extra for seat reservations?

I’m flying to Berlin then getting a train to Brussels, does this need a reservation do you know? It doesn’t say I do in the app but when I look on the DB website at the exact train journey it’s 81€. How am I supposed to reserve the train (on any journey that require a reservation) as I don’t see an option for having an interrail pass? 
 

Are there train journeys from Brussels-Paris, Paris-Zurich, Venice-Rome which don’t need a reservation (I don’t mind if the journey is longer) as the app says no reservation so is this possible? 

There are just some countries and / or some trains where you'll need a seat reservation. 

From Berlin to Bruxelles: no reservation needed. But you can't go by Thalys train (where you'd need a reservation).

But it's possible to do a reservation. No need to say at DB that you have an interrail pass. 

For the rest of your journey: It's completely possible to do the whole journey without mandatory reservation. If time doesn't matter... 

Thank you. How do I navigate the trains and also avoid booking a reservation. When I select ‘no reservation’ and ‘interrail pass’ on the planner app it gives me a journey for each destination but some Poole are still saying that I need to reserve? However, if I got to the station and it turns out that I need a reservation would I be able to book it there and then? 

If you click on "Seat only”, you can have a 4€ reservation for the complete journey Berlin - Brussels. It's optional, but wise on Friday and Sunday evening.