You'll need at least one 20 ampere circuit to supply the bathroom receptacles, which cannot serve any other rooms.
You cannot use 14 AWG conductors anywhere on a 20 ampere circuit, all the conductors must be 12 AWG or larger.
The likely course is to have one GFCI protected 20 ampere circuit to serve the bathroom receptacles. One 15 or 20 ampere circuit to supply the lighting in the bathroom (this will include the exhaust fan). And one 15 or 20 ampere circuit to supply the refrigerator. Later when you want to add the microwave, you can install another 15 ampere circuit to supply it.
First, check out my answer to "What is a double-stuff breaker?"
Second, take a quick moment to educate yourself on "rule of six" panels. I am fairly sure you don't have one.
single pole vs tandem locations
Generally, common 1-pole breakers can go anywhere in the panel (except in Rule of Six panels in the top section, because that would put you over six).
Double-stuff breakers can only go in spaces designed for double-stuffs. A typical panel will allow this in 0, some or all of its spaces -- though "all" is becoming rare. The labeling on the panel will say where they are allowed. Newer panels also have "keying" which makes the wrong breaker impossible, though older ones do not, and there's always the wiseguy with a file defeating the keying.
do you use single until you upgrade to tandem?
Tandem is a downgrade. The breaker will perform more poorly and high loading on one side will make the other side more susceptible to nuisance trip. Much worse, double-stuffs are not available at all in AFCI or GFCI, and virtually every residential circuit today requires those. What's more, remodeling must be to new codes and that requires more circuits.
Double-stuffs are the mark of a cheap builder who decided to save $30 by buying a 20-space panel instead of the 42-space the house actually needs.
what does "circuit breaker was oversized" mean?
The breaker's job is to protect wires and devices. 14AWG wire needs 15A protection or it will overheat. Common receptacles need 20A protection or they will overheat. Certain devices need protection of a size they will specify.
An oversized breaker means somebody made a mistake, or swapped in a too-large breaker because he was overloading the circuit and got sick of the breaker tripping. Replace the correct breaker. Breakers are $5.
what's a normal breaker really called?
It's more defined by the absence of a label like duplex or quadplex. To be more specific, call it a 1-pole breaker or 2-pole breaker.
general rules?
Yah. Only use breakers listed for that panel. I certainly hope your panel is a Siemens/ITE because almost all your breakers are listed for those panels.
The Square D Homeline breaker in position 17 is the snake in the stick pile.
Now a lot of people go "oh, but they fit". Those people aren't qualified to decide whether they fit, that's UL's job. There are several brands of 1" breaker that will mutually snap in, but if you have any experience you'll notice the insertion force is all wrong. That's because the busbar blades are each shaped differently, and each breaker is made to mate with its family's busbar and not the others.
There is such a thing as a "classified breaker" made by one brand specifically for another brand's panel, but if square D made such a thing for Siemens, they would never market it as Homeline (HOM).
Best Answer
Your planning is a bit...off
Thailand, like most places in the world that aren't in North America, uses IEC style nomenclature and wiring, from what I can tell (the actual standards aren't available in English, but the two threads I found on the topic on a Thai expat forum are thoroughly steeped in IEC terminology, such as RCD/RCBO, metric wire sizing, 400Y/230 LV distribution everywhere, and so on). As a result, 15A isn't even considered a standard breaker size, and Thai wiring rules apparently limit 1.5mm² to 10A anyway, with 16A circuits requiring 2.5mm² wire instead.
Given this, and the fact that Thailand, while having a legacy of using NEMA 1-15/5-15 plugs/receptacles (which is wrong, since those are a 120V config, not 220-240V), now uses its own plug standard (considered a "Type O" in the IEC taxonomy) rated for 16A/230V, you'll want to run your general appliance branch circuits as 16A circuits with 2.5mm² wire. You may be able to use 10A circuits for lighting, though, so do check with someone locally knowledgeable about that.
Wet area receptacles (kitchen/bath/outside) as well as water-heater type appliances will need RCD protection, although it may or may not required panel-wide, best I can tell. Eurostyle consumer units with DIN rail breakers may be permissible, although if you're going above about 24 breaker spaces, special IEC-rated versions of North American panelboard construction are available and may be preferable to the more traditional "consumer unit" construction.
Likewise, your service sizing will be...small by US standards. It seems that 30/32A and 50A are common service sizes in Thailand, with larger services delivered as three-phase. Speaking of large loads, note that that 2kW water heater will be perfectly happy on a 16A circuit, although the air conditioner may or may not require something larger.
Other than that, and any provisioning requirements the Thai standards provide, I'd provision:
Last but not least, Thailand apparently uses TN-C-S grounding, similar to what we do over here in North America.