Can i cross train hard in between hard days?

Take it easy.

Although your legs and structural muscles may feel fine by the effort, you have to remember your heart is also a muscle and fatigues just as well to overuse. You can determine if you are overtraining your heart by keeping a log of your resting HR after waking up--any spike in HR can signal overtraining and will warrant some recovery time.

No, this is a classic rookie mistake. Just because it is non impact doesnt mean you can have a free pass. Cortisol levels will still spike and you will still need time for metabolic recovery. This being said, there are some exercises that runners will find inherently more difficult such as box jumps, swiming or weight lifting, which would be benificial to do on the easy days.

Use common sense

so basically i shouldn't do hard intervals, and keep it relatively easy.

If in your 20's for sure.

If in your early 30's probably.

If in your late 30's probably not

If 40+ no

Too much density of training on your running days. And cross training is an excuse not to run. Do it only when injured. Runners run.

Don't waste your time.

If you want to be BUFF, stop running.

If you want to be RIPPED, stop running.

If you want to be both, start CrossFit.

Even The Rock adopted some CrossFit training methodology (see his twitter).

Of course you can. The heart can withstand the training better than your legs, so swimming is perfect. Your body will adapt. Go for it.

Of course you can.

Cyclists, swimmers, weight lifters, rowers and some runners go hard multiple days in a row.

The thing is, you will eventually have to recover. So, you could go "hard" Monday,Tuesday and Wednesday then take Thursday off and Friday very easy. Stress and recovery is not an exact science...