Thank you for the great questions, SwedishBear. Answers below
1. We will have 20 cards that we draw from, right?
This is the current deck total being used, yes, and it appears to be a solid number. We'll learn more through play-testing, but I imagine the range will be 15-30 cards and we'll most likely settle on 20.
2. Then we make our deck, can we include any mix of cards that we have acquired?
Deck building restrictions (including the limit number of individual cards and which spell groups can be included) have not yet been determined. One possibility is restricting colors based on the chosen Summoner. Another idea that I like better is allowing any cards the player has acquired, but granting benefits when players choose focused groups. This can be done simply by giving Summoners an advantage, such as "Wizard grants a 1 mana cost discount to all Arcane spells". It doesn't stop you from using the other spell groups, but it incentivizes Arcane if you pick the Wizard. Another way to restrict card types might be a hard rule, such as "If you include more than 2 spell groups, you cannot take cards with mana value higher than 4", but I personally lean towards the bonus model rather than restrictive model.
3. What information are saved for each character? For example, attack value, hitpoints, abilities like flying. Will gender be saved? Iím thinking on a succubus who converts male humanoids.
Characters store Power (attack value), Life (hit points), abilities (ex: Flying, Shield Block, etc.), status effects (ex: Frozen, Poisoned) and traits (types/groups). Traits are represented as hash-tagged labels at the bottom of the card and they indicate which groups the character belongs to for abilities that affect a group (ex:#skeleton, #ironclad, #dragon). Units can belong to multiple groups For example Skeleton Dragon is both #skeleton and #dragon and will be affected by cards that do things with skeletons and dragons. Gender (if we were to store this) would be a trait. Since the list of potential traits can become quite long, I wouldn't make something a trait unless there are several cards/abilities designed to affect that trait.