The question is for when you wanna reserve? As most trains open just 90 or 60days in advance and ontop not all Railcompanies haven´t published their Summertimetable for journey´s after the 12 June.

Paris - Zürich no Nighttrain just a direct Highspeed service (To save money i recommend to catch a Highspeed service just till the border Muhlhouse and from there with a regional train to Basel and further with any train to Zürich)
Direct train reservation will be up to 56€ the money saving route with 2 changes will be 10€ in advance or 20€ last minute

Zürich - Bled can be reserved via oebb.at Here a manual how to do :)

Zagreb - Budapest no Nighttrain just a daytime Eurocity reservations can be done via oebb.at (Check the daytime manual from the link above)

Vienna -Krakow can be reserved via oebb.at similar to Zürich - Bled :)

Krakow - Prague can be reserved via cd.CZ (Czech Rail or you use the service of POLRAIL a polish ticket agency)

Note that all trains labelled as NightJet (=night trains mostly to/fro/via Austria) there is just a small discount for passholders-around 15€. You have to pay substantial supplmts for using real bed space in them=sometimes ven more as a night in a hoStel would cost

There is no discount it are fixed prices from 14€ for a seat up to 150€ for a single deluxe sleeüer  And Between 150€ for a Single Deluxe Sleeper with Eurail/Interrail and a last minute booking of 220€ for the same is quite a lot more than 15€ Just a example what reservation fee´s can be expected on Nightjets https://rail.cc/interrail-night-train/vienna-paris-oebb-nightjet-nj468/634

Great thank you! I was planning on booking as soon as possible for 16th May- and 4 weeks onwards. With the money saving route from Paris- Zurich, does this come in the interrail pass and if so does it count as 2 travel days or just 1? 

Yes the Route Paris - Muhlhouse - Basel - Zürich comes to the travelpass and takes one travelday if you do this journey on the same day :)

On a travelday you can travel as much you like/manage in Europe :)