I do mahasi technique and in it you should balance your walking time with your sitting time - they both should be the same time

1 .  what are all the reasons for this ?

2 . how other vipassana techniques like goneka manage without walking meditation (or at least not an equal amount of walking andsitting time)

3 . and my main question : if i do samatha meditation plus the regular vipassana one - and i do just sitting in the samatha practice would it hurt the balance of the vipassana practice ? and why you think it will or will not affect the balance ?

I think the idea is to practice all the time.  On a formal retreat they do an equal balance of walking and sitting so you don't have to stop practicing all day.  It wouldn't be very healthy to sit for long periods, and it would cause extra physical distraction with body pain.  Some traditions try to make use of this kind of aversion and physical pain, but it isn't necessary.

If you're just practicing at home, try doing your method whenever it's possible during the day - driving, walking, doing chores, eating, falling asleep, taking a shower, etc.

What about balancing restlesness and concentration ? 

im suppose to do same amount of walking and sitting even if i only do 5 minutes of walking and five of sitting

I know about mindfulness in daily life but the balancing of walking and sitting in formal meditation seems like an important issue in my technique

Not sure if this directly answers your question, but there is a related sutta: Aggi sutta from tbe Samyutta Nikaya

The reason for walking and sitting in the Mahasi Sayadaw tradition is balancing the factors energy and concentration. When there is to much energy, there will be restlesness. When there is to much concentration, there will be sleepinesss. The alternation between sitting and walking is meant for retreat situations. In daily life you don't have to alternate sitting and walking because the energy factor is usually higher then the concentration factor. That's why doing sitting is recommended, so that you boost your concentration. So don't worry about how many minutes sitting and how many minutes walking. Mindfulness is the factor, that can balance all the other factors. So if you do your best to be mindful as much as possible then energy and concentration will be balanced.

As others have suggested, it's not a requirement to practice one for one amounts of sitting and walking meditation. However, I will offer up the following observations:

• Alternating periods of walking and sitting allows for the knees to get a bit of a rest. If you're not used to sitting 45-60 mins at a time and do this for 10-12 hours, your knees are going to feel it. This is pretty typical during retreats.
• You can bring the concentration/observational momentum gained during one into the other while ultimately trying to maintain continuity. This is a pretty valuable skill in and of itself. When you can sit down and elicit the jhanic fireworks, but can't do this while standing or walking, it begs the question, why not? Which then might lead one to ask, how now? Ditto with regards to continuity of mindfulness while transitioning from sitting to standing, from standing to walking, walking back to standing, back down to sitting. Consider this takes place over the period of (let's say) 100 minutes.

Tangentially, in the book, In This Very Life, Ven. Sayadaw U Pandita suggests the following: