No, don't go in with a frag vest. They are designed to be fitted with a t-shirt.
A lot of people have mixed opinions abou the new ruck, but I will say this.
The ruck is great. It works well with the tacvest and the frag vest. However, it requires a lot more adjustment than the old one. You will have to spend at least 3-4 13km rucks to get it adjusted properly.
I can vouch for the new ruck, as I did the Ironman with it. It fitted fine and I can honestly say that it is better than the '64 pattern.
This being said, a couple of pointers for the new ruck:
-Make sure the weight is evenly proportioned. You must pay extra attention to how you load it, otherwise it will be uncomfortable.
-If you are going on extended marches, it is advisable to move the compression sack complete into the upper compartment. On this ruck, it is even MORE important to have the heavy stuff at the top of the ruck.
-Adjust, adjust and adjust. This ruck is like going from a '87 Mazda B2000 to an Audi. It is a lot more finicky, but will produce the optimum results.
-You MUST take the pressure off the tension bars each and every time you remove the ruck and then retighten them whe you put them back on.
-Play with the arch supports. I found the models that they use to give you a baseline on how to shape your arch supports are a little to extravagent. Try making the arches less extreme.
-Adjust the waist belt up or down using the velcro until it fits right on your hips. You will also need to wear your tacvest correctly, for all the ganstas
that like to wear it low.
Hope this helps a little!