I once weigh 55kg at 169cm. Now at 66kg aiming for 70kg.

You're ectomorph. Avoid cardio at all cost. It will hurt your gains. The compound work is a good step forward. But nutrition is more important than the training.. especially for hardgainers.

With the weight you are now, i would avoid cardio. You can do cardio, but not now. I am doing circuit training myself, its a form of cardio..but that's because i see my tummy growing. Most i would do is twice week on off training days, and that is only 20minutes per session.

But at your weight 55kg, i would not even bother. Gaining fat would be the least of my concern.. In fact, at 55kg..u will look better if u gain some fat. The most important thing u need to do is to increase your calories. Meaning, u have to eat more. Add some extra stuff in your diet, say for example try eating 2 whole eggs per day. Try increase the intake to 3 eggs, then 4. Then u can add more food on top of that, add tuna, beef, kidney beans, chicken... Its not easy being consistent on the diet. But if you're persistent with eating every 3 hours, you'll be in the right track. Nutrition is key.

If you consume more calories than you need, eventually u will gain weight. If you're taking less calories than what your body needs..eventually you'll loose weight. What your body needs depends on your activity level and your metabolic rate. Since cardio increases the metabolic rate, on top of the already high metabolic rate of the ectomorph; no wonder you're loosing weight. Educate yourself about the ectomorph.

Its hard to gain lean muscle mass without gaining some fat. Unless you're genetically gifted, its next to impossible. Its easier to bulk first..then cut.
The advantage of being a beginner is that you have the drive. Use that drive in planning your meals, on increasing the intensity of your workout.. Remember that nutrition comes first, training is to stimulate growth. Growth comes from resting and proper nutrition.

Do whatever you want, its your workout. Though yeah_guy have a valid point on Rippetoe... Once you educate yourself about bodybuilding you will understand his point of view.

Cardio before your workout will drain your energy on the main lifts.
If you insisted on doing cardio, do it on Off lifting days.

Just dishing out my 2 cent.