1.you could use any of the following veggies-ash gourd,yellow pumpkin,snake gourd,big yellow cucumber(called vellarikkai in tamil)maybe you could even try with zuchini.
2.amy dal such as chana dal,mung dal or even tuvar dal though it is not used much in kootu.
3.a paste of grated fresh coconut,green chillies/red chillies with or without cumin.This coconut paste is optional and used more in kerala iyer type cooking.
SO to assemble your kootu,dice veggie of your choice,add one of the above mentioned dals,veggies should be more in volume than dal,pressure cook them together.When cooked,add salt and turmeric powder,let boil once and season with mustard seeds,urad dal, 1-2 broken red chillies,curry leaves if you have,and maybe 1tbsp of fresh grated coconut which you fry to a golden brown in the seasoning oil.
To serve 4 people you will need about 500gms-750 gms of any of these veggies.Add 3tbsp this quantiity of channa dal or 2-3 tbsp of mung dal for this quantiity.If you are using coconut paste,grind 3-4 tbsp fresh coconut with 2 green chillies wiht 1/4 tsp cumin and add after the dal veggie is cooked.
Hope this helps you.I' ll be glad to know if your recipe came out well and hope you have a good time with your guests.