Starting a Mediterranean garden in a small space

I have an l shaped built planting area which is 3m long each side but only about 50cm wide in terms of planting space. It is in a relatively sunny part of my east-west garden getting sunlight late morning for the rest of the day. I have always found it tricky to get the rightplants as there is a huge tree in the neighbours garden which sucks out moisture. Osteospernem and lavender have done well. Given it is dry and sunny I thought I might convert to a Mediterranean style. I have some agaves inputs and yuccas which will go in although I appreciate they may outgrow it. I also have some arums that I think will do well there
Basically can someone advise what I should do to the soil eg what compost to condition it, do I need to add gravel as it will be fast draining anyway? Any suggestions for compact plants? Also I will put pots around the ares to give it a more court yard feel, any recommendations for compost for these plants or will multipurpose do? My agaves and yuccas seem ok so far with little special attention but I am not sure when I should pot them on to bigger pots.
Any advice welcome, my normal source of advice (mam) is not used to this style so we are struggling. Thanks!